Space Switching: Space Bottom Sheet Empty States

[ericdecanini]

When there are no spaces at the root or children of the current space, we want to show an empty state to notify the user. Designs of this are TBD.

As per https://github.com/vector-im/element-android/issues/6771, we also want to make the toolbar non-collapsible in an empty state.

Figma design: https://www.figma.com/file/Hw4gjP3pknMZUir8jQqmWH/%5BMobile%5D-IA-early-proposals?node-id=2523%3A135211

Parent epic: https://github.com/vector-im/element-android/issues/6605